Recognition Or Identification In Optical Correlators

Abstract

Optical correlator is an optical system able to create in the output the correlation of images of introduced optical signals. Correlation function is used as a decisive function in signal identification process. Often we are rather interested in recognizing than in exact identification of the optical signals. At first we must characterize signal features valid for recognition. Beyond a lot of signal characteristics we can discern two: the signal shape and contour sharpness (image contrast). Because the identification is carried out in the Fourier plane of ptical system we will define these features in spatial frequencies. So we discern: 1. The set of spatial frequency directions realized in the Fourier spectrum of the signal and 2. maximal spatial frequencies occouring in the Fourier spectrum. The first is connected with the shape and the second with the sharpness of contours in the input signal. The question is how will the correlator system respond to this characteristics? We tested two correlator systems: Van der Lugt correlator and quasi coherent correlator. The optical signals used in the examinations were intentionally constructed in order to emphasize the above mentioned characteristics of them, There wert: two diffraction gratings with the same grating constants and modulation deepths. They differed in the progress of the transmittance only. The tested correlators were arranged in known manner. Light intensity collected in the center of correlation image in the output of correlator system served as identity gauge of tested signals. Conclusions: In the experiments it occoured that the Van der Lugt correlator is more sensible device than the quasi in transmittance progresses can be detected sharply. The nonlinear recording of the matched filter promotes this effect. In the contrary because of better linearity of recording the matched filter and the scattering nature of diffuse illumination, the quasi coherent correlator is sensible only for the general shape of the signal. For identifying optical signals exactly, with full characteristic, the Van der Lugt correlator is unexchangeable. But when one is interested in recognizing signal shapes it is better to use the quasi coherent correlator.